i-adjective
hard to read; difficult to read
Used when text is physically hard to decipher (small, messy, complex characters) or when content is hard to understand.
この本は字が小さくて読みづらい。
This book is hard to read because the print is small.
彼の字は汚くて読みづらい。
His handwriting is messy and hard to read.
Standard spelling using づらい for 'difficult to do'.
Alternate kanji spelling; 辛い can also be read からい or つらい, so 読みづらい is clearer.
読みにくい is a more general 'hard to read' (objective difficulty), while 読みづらい often implies personal struggle or discomfort in reading.
Compound of 読み (reading) + づらい (difficult to do). The suffix づらい attaches to the masu-stem of verbs to express that an action is hard to perform due to physical or psychological difficulty.